Methods for producing hydrocarbons from sub-sea wells are known. Typically, hydrocarbons are produced from one or more sub-sea wellheads positioned at a sea-bed location that is some distance away from the location of an offshore drilling and production platform. The hydrocarbons produced at the sub-sea wellhead(s) are delivered to the offshore drilling and production platform through one or more pipelines. Alternatively, the hydrocarbons produced from sub-sea wells may be routed from a sub-sea wellhead directly to land through a pipeline, as long as the wellhead is located sufficiently close to the shore. Hydrocarbons may also be produced from a sub-sea well and then transferred through a pipeline to a moored floating production, storage, and offloading tanker, commonly referred to in the art as an FPSO tanker.
Natural gas from either production reservoirs or storage reservoirs typically contains water, as well as other contaminant species, which form solids during the liquefaction to produce liquefied natural gas (LNG). It is common practice for the natural gas to be subjected to a dehydration process prior to transfer or liquefaction. Dehydration of natural gas produced from sub-sea wells is conducted to reduce the risk of corrosion and free-water accumulation in the low points of a pipeline.
Methods of dehydrating natural gas feed streams known in the art include absorption of water in glycol (ethylene, diethylene, triethylene and tetraethylene), using solid desiccants such as silica gel, silica-alumina gels, calcium chloride [CaCl2], or hydrated aluminium oxide, or using molecular sieves. Molecular sieves are used when very low water dew points are required. Another process used to simultaneously remove water and heavy hydrocarbons for dew point control is dry bed adsorption, a process by which a gas stream is passed through adsorbing dry beds of, typically, silica gel.
Traditional dehydration processes described above were all developed to specifically prevent gas hydrate formation from occurring in pipelines and heat-exchangers upstream of a liquefaction vessel. Hydrates are stable crystalline solids having the outward appearance of ice with gas molecules trapped in an ice-like cage structure. The general term for this type of solid is clathrate. Most gases can form hydrates. It is known that C1 to C4 hydrocarbons as well as hydrogen sulphide (H2S) and carbon dioxide (CO2) readily form hydrates at low temperature and high pressure. When the temperature is raised above the so-called “hydrate dissolution temperature” the hydrates dissolve or meet, liberating the trapped gas and producing a water-containing liquid.
The formation of gas hydrates is thermodynamically suppressed by adding antifreeze materials such as salts or glycols and/or inhibiting nucleation and growth of hydrates by using particular polymers or surfactants. Moreover these existing dehydration processes are costly and complex and ill-adapted to use at the sea-bed.
The present invention was developed to provide a process and apparatus for dehydrating natural gas in a sub-sea environment.